Water-dispersed pressure-sensitive adhesive composition, pressure-sensitive adhesive and pressure-sensitive adhesive sheet

ABSTRACT

The present invention makes a water-dispersed PSA composition capable of forming a PSA that exhibits good adhesiveness to a non-polar adherend such as polyethylene and polypropylene at a low temperature. The water-dispersed PSA composition comprises a (meth)acrylic polymer obtained by polymerizing a monomer composition comprising as a primary component an alkyl(meth)acrylate having an alkyl group with 1 to 18 carbon atoms, and a latex containing a rubber component that is immiscible with the (meth)acrylic polymer, the (meth)acrylic polymer and the latex being mixed at a weight ratio ((meth)acrylic polymer/rubber latex) within a range of 95/5 to 25/75 based on solid contents; wherein a storage modulus in a range of −15° C. to 25° C. is 1 MPa or smaller, the storage modulus being determined by a dynamic viscoelastic measurement in which shear strain is applied at a frequency of 1 Hz, the measurement being taken with respect to the water-dispersed PSA composition dried at 100° C. for 3 minutes.

TECHNICAL FIELD

The present invention relates to a water-dispersed pressure-sensitiveadhesive composition used for pressure-sensitive adhesives, etc., aswell as a pressure-sensitive adhesive and a pressure-sensitive adhesivesheet each formed of the composition. The present application claimspriority to Japanese Patent Application No. 2011-117951 filed on May 26,2011, Japanese Patent Application No. 2011-160387 filed on Jul. 22,2011, and Japanese Patent Application No. 2012-033057 filed on Feb. 17,2012, and the entire contents of these applications are incorporated inthe present application as reference.

BACKGROUND ART

In recent years, from the standpoint of environmental stress, reductionof organic solvent usage has been desired. With respect to apressure-sensitive adhesive (PSA) composition laminated on a PSA sheet,it has been also underway to shift from solvent-based PSA compositionswhere an organic solvent is used as the solvating media towater-dispersed PSA compositions where water is used as the dispersionmedia.

As such a PSA, an acrylic PSA has been widely used because of itsversatility. However, there exists a problem such that an acrylic PSA,in general, is poorly adhesive to a non-polar adherend such aspolyethylene and polypropylene.

In order to solve this kind of problem, in a disclosed method, atackifying resin having a high softening point is added (e.g., seePatent Document 1). Also disclosed is a PSA that comprises an acrylicpolymer and a rubber-based PSA, and further comprises a rosin-basedtackifying resin, whereby the PSA exhibits good adhesiveness to anon-polar adherend even at a low temperature (e.g., see Patent Document2).

CITATION LIST [Patent Literatures]

[Patent Document 1] Japanese Patent Application Publication No. H3-34786

[Patent Document 2] Japanese Patent Application Publication No.2008-163095 SUMMARY OF INVENTION Technical Problem

What has been desired is, however, a water-dispersed PSA compositioncapable of forming a PSA having yet better adhesive strength to anon-polar adherend at a low temperature than the emulsion-based PSA'sdescribed in Patent Documents 1 and 2.

An objective of the present invention is to provide a water-dispersedPSA composition capable of forming a PSA that exhibits greateradhesiveness to a non-polar adherend such as polyethylene andpolypropylene at a low temperature.

Solution to Problem

The present inventors have earnestly researched in order to solve theproblem. As a result, they discovered that with a water-dispersed PSAcomposition formed to comprise a (meth)acrylic polymer and a latex thatis immiscible with the (meth)acrylic polymer at a prescribed weightratio so that the PSA after dried would exhibit a prescribed storagemodulus, a PSA exhibiting good adhesiveness to a non-polar adherend at alow temperature could be formed; and completed the present invention.

It is considered that since the water-dispersed PSA compositionaccording to Patent Document 2 uses a rubber component that exhibits ahigh modulus of elasticity in a low temperature range, its storagemodulus exceeds 1 MPa in a range of −15° C. to 25° C., resulting ininsufficient wettability to adherends; and therefore, it exhibits pooreradhesiveness.

More specifically, the water-dispersed PSA composition according to thepresent invention comprises a (meth)acrylic polymer obtained bypolymerizing a monomer composition comprising as a primary component analkyl(meth)acrylate having an alkyl group with 1 to 18 carbon atoms, anda latex containing a rubber component that is immiscible with the(meth)acrylic polymer, the (meth)acrylic polymer and the latex beingmixed at a weight ratio ((meth)acrylic polymer/rubber latex) within arange of 95/5 to 25/75 based on solid contents, wherein a storagemodulus in a range of −15° C. to 25° C. is 1 MPa or smaller, the storagemodulus being determined by a dynamic viscoelastic measurement in whichshear strain is applied at a frequency of 1 Hz, the measurement beingtaken with respect to the water-dispersed PSA composition dried at 100°C. for 3 minutes.

In the water-dispersed PSA composition according to the presentinvention, the latex containing the rubber component preferably has aloss tangent peak temperature of −5° C. or below when determined by adynamic viscoelastic measurement in which shear strain is applied at afrequency of 1 Hz.

It is preferable that the water-dispersed PSA composition according tothe present invention further comprises a tackifier that is misciblewith the rubber component, but immiscible with the (meth)acylic polymer.

The water-dispersed PSA composition according to the present inventioncomprises the tackifier preferably in an amount within a range above 0part by weight up to 100 parts by weight relative to 100 parts by weightof solid contents of the rubber latex.

In the water-dispersed PSA composition according to the presentinvention, the tackifier preferably has a softening point of 80° C. to150° C.

In the water-dispersed PSA composition according to the presentinvention, it is preferable that the rubber component contained in thelatex is at least one species selected from a group consisting ofnatural rubber, synthesized polyisoprene rubber, polybutadiene rubber,and styrene-butadiene rubber.

The PSA according to the present invention is characterized by beingformed with a water-dispersed PSA composition according to the presentinvention described above.

The PSA sheet according to the present invention is characterized bycomprising a PSA layer formed with a water-dispersed PSA compositionaccording to the present invention described above.

Advantageous Effects of Invention

According to the present invention, can be provided a water-dispersedPSA composition capable of forming a PSA that exhibits good adhesivenessto a non-polar adherend such as polyethylene and polypropylene at a lowtemperature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a graph indicating the evaluation results on themiscibility of the (meth)acrylic polymer and the rubber component usedin Example 3.

FIG. 2 shows a TEM image of the PSA sheet obtained in Example 3.

FIG. 3 shows cross-sectional diagrams schematically illustrating outlineconfigurations of PSA sheets according to some embodiments of thepresent invention.

EMBODIMENTS OF INVENTION

Particulars of the present invention are described below.

It is noted that “A to B” indicating a range means “A or greater, but Bor smaller” and various physical properties given in the presentdescription refer to values measured by the methods described inExamples described later unless otherwise noted. In the presentdescription, “(meth)acryl” in “(meth)acrylic polymer” and the like means“acryl and/or methacryl”. In addition, “water-dispersed” in the presentdescription refers to a state where components are at least partiallydispersed in water; and for instance, “water-dispersed PSA composition”refers to a composition comprising a PSA composition and water and beingin a state where the PSA composition is partially dispersed in water. Itis noted that the term “dispersed” refers to a state where at least partof components are not dissolved in water, including also a suspendedstate and an emulsified state.

(I) Water-Dispersed PSA Composition

The water-dispersed PSA composition according to the present inventioncomprises a (meth)acrylic polymer obtained by polymerizing a monomercomposition comprising as a primary component an alkyl(meth)acrylatehaving an alkyl group with 1 to 18 carbon atoms, and a latex containinga rubber component that is immiscible with the (meth)acrylic polymer. Inthe water-dispsersed PSA composition according to the present invention,the (meth)acrylic polymer and the latex are mixed at a weight ratio((meth)acrylic polymer/rubber latex) within a range of 95/5 to 25/75based on solid contents. It has a storage modulus of 1 MPa or smallerwhen determined by a dynamic viscoelastic measurement in which shearstrain is applied at a frequency of 1 Hz, with the measurement beingtaken with respect to the water-dispersed PSA composition dried at 100°C. for three minutes.

The respective components are described below.

(i) (Meth)acrylic Polymer

The (meth)acrylic polymer can be obtained by polymerizing a monomercomposition comprising, as a primary component, an alkyl(meth)acrylatehaving an alkyl group with 1 to 18 carbon atoms. The monomer compositionmay comprise, as desired, a functional-group-containing unsaturatedmonomer or an unsaturated monomer copolymerizable with thealkyl(meth)acrylate or the functional-group-containing unsaturatedmonomer.

The alkyl(meth)acrylate having an alkyl group with 1 to 18 carbon atomsincludes compounds represented by the following general formula (1):

[Chem. 1]

H₂C═CR¹COOR²  (1)

(in general formula (1), R¹ is a hydrogen atom or a methyl group and R²is an straight-chained or branched alkyl group having 1 to 18 carbonatoms.)

Specific examples of R² in general formula (1) include methyl group,ethyl group, propyl group, isopropyl group, butyl group, isobutyl group,sec-butyl group, t-butyl group, pentyl group, neopentyl group, isoamylgroup, hexyl group, heptyl group, octyl group, 2-ethylhexyl group,isooctyl group, nonyl group, isononyl group, decyl group, isodecylgroup, undecyl group, dodecyl group, tridecyl group, tetradecyl group,pentadecyl group, hexadecyl group, heptadecyl group, octadecyl group,and so on.

Specific examples of the alkyl(meth)acrylate represented by generalformula (1) include methyl(meth)acrylate, ethyl(meth)acrylate,propyl(meth)acrylate, isopropyl(meth)acrylate, butyl(meth)acrylate,isobutyl(meth)acrylate, sec-butyl(meth)acrylate, t-butyl(meth)acrylate,pentyl(meth)acrylate, neopentyl(meth)acrylate, isoamyl(meth)acrylate,hexyl(meth)acrylate, heptyl(meth)acrylate, octyl(meth)acrylate,2-ethylhexyl(meth)acrylate, isooctyl(meth)acrylate, nonyl(meth)acrylate,isononyl(meth)acrylate, decyl(meth)acrylate, isodecyl(meth)acrylate,undecyl(meth)acrylate, dodecyl(meth)acrylate, tridecyl(meth)acrylate,tetradecyl(meth)acrylate, pentadecyl(meth)acrylate,hexadecyl(meth)acrylate, heptadecyl(meth)acrylate,octadecyl(meth)acrylate, and so on. These alkyl(meth)acrylates can beused singly or in combination of two or more kinds.

The number of carbon atoms of the alkyl group in the alkyl(meth)acrylateis preferably 2 to 18, or more preferably 4 to 12.

The amount of the alkyl(meth)acrylate to be added can be, for instance,60 to 99.5 parts by weight or preferably 70 to 99 parts by weightrelative to 100 parts by weight of a total amount of the monomercomposition.

Examples of the functional-group-containing unsaturated monomer includecarboxyl-group-containing unsaturated monomers.

Carboxyl-group-containing monomers include unsaturated carboxylic acidssuch as (meth)acrylic acid, itaconic acid, maleic acid, fumaric acid,crotonic acid, cinammic acid, etc.; monoesters of unsaturateddicarboxylic acids such as monomethyl itaconate, monobutyl itaconate,2-acryloyloxyethyl phthalate, etc.; monoesters of unsaturatedtricarboxylic acids such as 2-methacryloyloxyethyl trimellitate,2-methacryloyloxyethyl pyromellitate, etc.; carboxyalkyl acrylates suchas carboxyethyl acrylates (β-carboxyethyl acrylate, etc.), carboxypentylacrylates, etc.; acrylic acid dimer, acrylic acid trimer; anhydrides ofunsaturated dicarboxylic acids such as itaconic acid anhydride, maleicacid anhydride, fumaric acid anhydride, etc.; and so on.

Examples of the functional-group-containing unsaturated monomer otherthan the carboxyl-group-containing unsaturated monomers includehydroxyl-group-containing unsaturated monomers such as 2-hydroxyethylacrylate, 2-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, etc.;amide-group-containing unsaturated monomers such as (meth)acrylamide,N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide,N-isopropyl(meth)acrylamide, N-butyl(meth)acrylamide,N-methoxymethyl(meth)acrylamide, N-methylol(meth)acrylamide,N-methylolpropane(meth)acrylamide, etc.; amino-group-containingunsaturated monomers such as aminoethyl(meth)acrylate,N,N-dimethylaminoethyl(meth)acrylate, t-butylaminoethyl(meth)acrylate,etc.; glycidyl-group-containing unsaturated monomers such asglycidyl(meth)acrylate, methylglycidyl(meth)acrylate, etc.;cyano-group-containing unsaturated monomers such as (meth)acrylonitrile,etc.; maleimide-group-containing monomers such as N-cyclohexylmaleimide, N-isopropyl maleimide, N-lauryl maleimide, N-phenylmaleimide, etc.; itaconimide-group-containing monomers such as N-methylitaconimide, N-ethyl itaconimide, N-butyl itaconimide, N-octylitaconimide, N-2-ethylhexylitaconimide, N-cyclohexyl itaconimide,N-lauryl itaconimide, etc.; succinimide-group-containing monomers suchas N-(meth)acryloyloxymethylene succinimide,N-(meth)acryloyl-6-oxyhexamethylene succinimide,N-(meth)acryloyl-8-oxyoctamethylene succinimide, etc.;vinyl-group-containing heterocyclic compounds such asN-vinylpyrrolidone, N-(1-methylvinyl) pyrrolidone, N-vinylpyridine,N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperazine,N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, N-vinyloxazole,N-vinylmorpholine, (meth)acryloylmorpholine, etc.;sulfonate-group-containing unsaturated monomers such as styrenesulfonate, allylsulfonate, 2-(meth)acrylamide-2-methyl propanesulfonate, (meth)acrylamide propane sulfonate,sulfopropyl(meth)acrylate, (meth)acryloxynaphthalene sulfonate, etc.;phosphate-group-containing unsaturated monomers such as2-hydroxyethylacryloyl phosphate, etc.; functional monomers such as2-methacryloyloxyethyl isocyanate, etc.; as well as N-vinyl carboxylicacid amides, and so on.

The amount of the functional-group-containing unsaturated monomer to beadded is, for instance, 0.5 to 12 parts by weight or preferably 1 to 10parts by weight relative to 100 parts by weight of a total amount of themonomer composition.

Examples of an unsaturated monomer copolymerizable with thealkyl(meth)acrylate and the functional-group-containing unsaturatedmonomers include vinylester-group-containing monomers such as vinylacetate, etc.; unsaturated aromatic monomers such as styrene,vinyltoluene, etc.; (meth)acrylic acid alicyclic hydrocarbon estermonomers such as cyclopentyl di(meth)acrylate, isobornyl(meth)acrylate,etc.; alkoxy-group-containing unsaturated monomers such asmethoxyethyl(meth)acrylate, ethoxyethyl(meth)acrylate, etc.; olefinicmonomers such as ethylene, propylene, isoprene, butadiene, isobutylene,etc.; vinyl-ether-based monomers such as vinyl ether, etc.;halogen-atom-containing unsaturated monomers such as vinyl chloride,etc.; and others such as tetrahydrofurfuryl(meth)acrylate, heterocycliccompounds of fluoro(meth)acrylates, etc., acrylic-acid-ester-basedmonomers containing a halogen atom, and so on.

The monomer composition may further comprise a multi-functional monomer.Examples of the multi-functional monomer include (mono or poly)ethyleneglycol di(meth)acrylates such as ethylene glycol di(meth)acrylate,diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, tetraethylene glycoldi(meth)acrylate, etc.; (mono or poly)alkylene glycol di(meth)acrylatessuch as (mono or poly)propylene glycol di(meth)acrylates, etc., such aspropylene glycol di(meth)acrylate, etc.; as well as (meth)acrylic acidesters of polyols such as neopentyl glycol di(meth)acrylate,1,6-hexane-diol di(meth)acrylate, tetramethylolmethanetri(meth)acrylate, pentaerythritol di(meth)acrylate, trimethylolpropanetri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritolhexa(meth)acrylate, etc.; divinylbenzene; and so on. Other Examples ofthe multi-functional monomer include epoxy acrylates, polyesteracrylates, urethane acrylates and the like.

The monomer composition may further comprise analkoxysilyl-group-containing vinyl monomer. Thealkoxysilyl-group-containing vinyl monomer includes silicone-based(meth)acrylate monomers, silicone-based vinyl monomers, and so on.

Examples of the silicone-based (meth)acrylate monomer include(meth)acryloxyalkyl-trialkoxysilanes such as(meth)acryloxymethyl-trimethoxysilane,(meth)acryloxymethyl-triethoxysilane,2-(meth)acryloxyethyl-trimethoxysilane,2-(meth)acryloxyethyl-triethoxysilane,3-(meth)acryloxypropyl-trimethoxysilane,3-(meth)acryloxypropyl-triethoxysilane,3-(meth)acryloxypropyl-tripropoxysilane,3-(meth)acryloxypropyl-triisopropoxysilane,3-(meth)acryloxypropyl-tributoxysilane, etc.; (meth)acryloxyalkylalkyl-dialkoxysilanes such as(meth)acryloxymethyl-methyldimethoxysilane,(meth)acryloxymethyl-methyldiethoxysilane,2-(meth)acryloxyethyl-methyldimethoxysilane,2-(meth)acryloxyethyl-methyldiethoxysilane,3-(meth)acryloxypropyl-methyldimethoxysilane,3-(meth)acryloxypropyl-methyldiethoxysilane,3-(meth)acryloxypropyl-methyldipropoxysilane,3-(meth)acryloxypropyl-methyldiisopropoxysilane,3-(meth)acryloxypropyl-methyldibutoxysilane,3-(meth)acryloxypropyl-ethyldimethoxysilane,3-(meth)acryloxypropyl-ethyldiethoxysilane,3-(meth)acryloxypropyl-ethyldipropoxysilane,3-(meth)acryloxypropyl-ethyldiisopropoxysilane,3-(meth)acryloxypropyl-ethyldibutoxysilane,3-(meth)acryloxypropyl-propyldimethoxysilane,3-(meth)acryloxypropyl-propyldiethoxysilane, etc.; and theircorresponding (meth)acryloxyalkyl-dialkyl(mono)alkoxysilanes; and so on.

Examples of the silicone-based vinyl monomer includevinyltrialkoxysilanes such as vinyltrimethoxysilane,vinyltriethoxysilane, vinyltripropoxysilane, vinyltriisopropoxysilane,vinyltributoxysilane, etc.; their correspondingvinylalkyldialkoxysilanes and vinyldialkylalkoxysilanes;vinylalkyltrialkoxysilanes such as vinylmethyltrimethoxysilane,vinylmethyltriethoxysilane, β-vinylethyltrimethoxysilane,β-vinylethyltriethoxysilane, γ-vinylpropyltrimethoxysilane,vinylpropyltriethoxysilane, γ-vinylpropyltripropoxysilane,γ-vinylpropyltriisopropoxysilane, γ-vinylpropyltributoxysilane, etc.;and their corresponding (vinylalkyl)alkyldialkoxysilanes and(vinylalkyl)dialkyl(mono)alkoxysilanes; and so on.

By using an alkoxysilyl-group-containing vinyl monomer, alkoxysilylgroups are introduced into the polymer chains and reactions among thesilyl groups allow formation of a crosslinked structure. Thesealkoxysilyl-group-containing vinyl monomers can be used singly or incombination, as appropriate.

The amount of these alkoxysilyl-group-containing vinyl monomers to beadded is, for instance, within a range greater than 0 part by weight upto 40 parts by weight or preferably within a range greater than 0 partby weight up to 30 parts by weight relative to 100 parts by weight ofthe alkyl(meth)acrylate.

The (meth)acrylic polymer according to the present invention can beobtained, for instance, by polymerizing the monomer compositiondescribed above by a polymerization method such as emulsionpolymerization, etc.

In emulsion polymerization, for instance, polymerization is carried outby suitably mixing in water a polymerization initiator, an emulsifier,and as necessary a chain transfer agent, etc., along with the monomercomposition. More specifically, for example, can be employed a knownemulsion polymerization method such as all-at-once supply method (onebatch polymerization method), monomer dropping method, monomer emulsiondropping method, etc. In monomer dropping method, can be suitablyselected either continuous dropping or portionwise dropping. Althoughthe reaction conditions, etc. can be appropriately selected, thepolymerization temperature is, for instance, 20° C. to 100° C.

The polymerization initiator is not particularly limited and apolymerization initiator usually used in emulsion polymerization can beused. Examples include azo-based initiators such as2,2′-azobisisobutyronitrile,2,2′-azobis(2-methylpropionamidine)disulfate salt,2,2′-azobis(2-methylpropionamidine)dihydrochloride salt,2,2′-azobis(2-amidinopropane)dihydrochloride salt,2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate,2,2′-azobis(N,N′-dimethylene isobutylamidine),2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride salt, etc.;persulfate salt-based initiators such as potassium persulfate, ammoniumpersulfate, etc.; peroxide-based initiators such as benzoyl peroxide,t-butyl hydroperoxide, hydrogen peroxide, etc.; substituted ethane-basedinitiators such as phenyl-substituted ethane, etc.; carbonyl-basedinitiators such as aromatic carbonyl compounds, etc.; redox-basedinitiators such as a combination of a persulfate salt and sodiumhydrogen sulfite, a combination of a peroxide and sodium ascorbate,etc.; and so on.

These polymerization initiators are used singly or in combination, asappropriate. Although the amount of the polymerization initiator to beadded can be suitably selected, it is, for instance, 0.005 to 1 part byweight or preferably 0.01 to 0.8 part by weight relative to 100 parts byweight of a total amount of the monomer composition.

The emulsifier is not particularly limited and an emulsifier usuallyused in emulsion polymerization can be used. Examples include anionicemulsifiers such as sodium lauryl sulfate, ammonium lauryl sulfate,sodium dodecyl benzene sulfonate, sodium polyoxyethylene lauryl sulfate,sodium polyoxyethylene alkyl ether sulfates, ammonium polyoxyethylenealkyl phenyl ether sulfates, sodium polyoxyethylene alkyl phenyl ethersulfates, sodium polyoxyethylene alkyl sulfosuccinates, etc.; non-ionicemulsifiers include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene aliphatic acid esters,polyoxyethylene-polyoxypropylene block polymers, etc.; and so on.Alternatively, may be used radically polymerizable (reactive)emulsifiers (e.g., HS-10 (available from Dai-Ichi Kogyo Seiyaku Co.,Ltd.)) having structures of these anionic emulsifiers or non-ionicemulsifiers in which radically polymerizable (reactive) groups such aspropenyl group, ally ether group, etc., have been introduced.

These emulsifiers can be used singly or in combination, as appropriate.The amount of the emulsifier to be added is, for instance, 0.2 to 10parts by weight or preferably 0.5 to 5 parts by weight relative to 100parts by weight of a total amount of the monomer composition.

A chain transfer agent adjusts the molecular weight of a polymer asnecessary and a chain transfer agent usually used in emulsionpolymerization is used. Examples include mercaptans such as1-dodecanethiol, mercaptoacetic acid, 2-mercaptoethanol, 2-ethylhexylthioglycolate, 2,3-dimethylmercapto-1-propanol, and so on. These chaintransfer agents are used singly or in combination, as appropriate. Theamount of the chain transfer agent to be added is, for instance, 0.001to 0.5 part by weight relative to 100 parts by weight of a total amountof the monomer composition.

In order to increase the stability of the (meth)acrylic polymer, forinstance, with aqueous ammonia, etc., it can be adjusted to, forexample, pH 7 to 9 or preferably to pH 7 to 8.

(ii) Latex Containing a Rubber Component

The water-dispersed PSA composition according to the present inventioncomprises a latex containing a rubber component that is immiscible withthe (meth)acrylic polymer (which hereinafter may be referred to as“rubber-based latex”).

The rubber component preferably has a loss tangent peak temperature of−5° C. or below when determined by a dynamic viscoelastic measurement inwhich shear strain is applied at a frequency of 1 Hz. The loss tangentpeak temperature of the rubber component is more preferably −10° C. orbelow, even more preferably −15° C. or below, even more preferably −20°C. or below, particularly preferably −25° C. or below, or mostpreferably −30° C. or below. By adjusting the loss tangent peaktemperature to be within the range described above, the modulus ofelasticity at a low temperature can be decreased and the adhesivestrength at a low temperature can be increased. Although the lower limitis not particularly limited for the loss tangent peak temperature of therubber component, it can be, for instance, −130° C. or above, preferably−120° C. or above, or more preferably −110° C. or above.

It is noted that the dynamic viscoelastic measurement performed withshear strain applied at a frequency of 1 Hz can be carried out by thesame method as in the “(5) Dynamic viscoelastic measurement of PSAsheet” in Examples described later.

Examples of the rubber-based latex include natural rubber latex andsynthetic rubber-based latex. Natural rubber latex can be modifiednatural rubbers obtained by grafting an alkyl(meth)acrylate, etc., on anatural rubber.

Synthetic rubber-based latex is an aqueous dispersion of a syntheticpolymer. The variety of synthetic polymers include polyisoprenes,styrene-butadiene copolymers (SBR),styrene-butadiene-vinylpyridine-based polymers, polybutadiene-basedpolymers, methyl-methacrylate-butadiene-based copolymers,acrylonitrile-butadiene-based polymers (NBR), polychloroprenes (CR), andso on.

It is noted that a person of ordinary skill in the art can figure outhow to obtain a latex containing a rubber component that has a losstangent peak temperature within the range described above and isimmiscible with the (meth)acrylic polymer, by modifying the compositionthrough adjustment of the compositional ratio (copolymerization ratio)of the rubber component based on the contents of description of thepresent application including the specific examples described later aswell as general technical knowledge such as solubility parameter (sp)values, etc.

As such a rubber-based latex, a commercial product can be used. Examplesof a styrene-butadiene latex include products available from ZeonCorporation (NIPOL series), JSR Corporation, Asahi Kasei ChemicalsCorporation, DIC Corporation (LACSTAR series), Nippon A&L Inc. (NALSTARseries), etc. Examples of styrene-butadiene-vinylpyridine latex includeproducts available from Zeon Corporation (NIPOL series), Nippon A&LCorporation (PYRATEX series), etc. Examples of polybutadiene latexinclude products available from Zeon Corporation (NIPOL series).Examples of MMA-polybutadiene latex include products available fromNippon A&L Corporation (NALSTAR series), etc. Examples ofacrylonitrile-butadiene latex include products available from ZeonCorporation (NIPOL series), Nippon A&L Corporation (CYATEX series), DICCorporation (LACSTAR series), etc. Examples of chloroprene latex includeproducts available from Showa Denko K. K. (SHOPRENE series), TosohCorporation (SKYPRENE series), etc.

Either one of these rubber-based latexes can be used as long as itsrubber component is immiscible with the (meth)acrylic polymer.

In a PSA formed of the water-dispersed PSA composition according to thepresent invention, the (meth)acrylic polymer and the rubber componentexist in immiscible phases. The miscibility of the (meth)acrylic polymercomponent and the rubber component can be evaluated by the peaks of the(meth)acrylic polymer component and the peaks of the rubber component inthe loss modulus curve and the loss tangent curve obtained by a dynamicviscoelastic measurement. More specifically, for instance, as shown inFIG. 1, when two peaks are present corresponding to a peak of a(meth)acrylic polymer and a peak of a rubber component in the lossmodulus (G″) curve or in the loss tangent (tan δ) curve obtained by adynamic viscoelastic measurement, it indicates that the (meth)acrylicpolymer component and the rubber component are immiscible with eachother.

It is noted that when the rubber component of the rubber-based latex andthe (meth)acrylic polymer are present in immiscible phases, in a driedPSA, the acrylic phase formed primarily of the (meth)acrylic polymer andthe rubber phase formed primarily of the rubber component form asea-island structure. Such a structure can be visually confirmed, forinstance, as shown in FIG. 2, by observing a section of dried PSA with atransmission electron microscope (TEM).

Hence, when the (meth)acrylic polymer peak and the rubber component peakoverlap with each other in the loss modulus curve and the loss tangentcurve obtained by a dynamic viscoelastic measurement, the miscibilitycan be evaluated by determining whether a sea-island structure has beenformed or not. For this case, in the present invention, as long as asea-island structure is formed to some extent, even if there is amiscible region, it is judged to be immiscible.

The weight ratio of such a (meth)acrylic polymer to the rubber-basedlatex ((meth)acrylic polymer/rubber-based latex) is preferably 95/5 to25/75 or preferably 90/10 to 30/70 based on their solid contents. Whenthe amount of the rubber component is smaller than the aforementionedrange, the adhesive strength to a non-polar adherend and the adhesivestrength at a low temperature will decrease. When the amount of theacrylic polymer component is smaller than the aforementioned range,sufficient adhesive strength to a polar adherend such as SUS, etc., maynot be obtained.

(iii) Tackifier

The water-dispersed PSA composition according to the present inventionpreferably comprises a tackifier (typically a tackifying resin) that ismiscible with the rubber component, but immiscible with the(meth)acrylic polymer.

Examples of such a tackifier include tackifying resins, with examplesincluding rosin-based resins such as rosin esters, hydrogenated rosinesters, disproportionated rosin esters, polymerized rosin esters, etc.;coumarone-indene-based resins such as coumarone-indene resins,hydrogenated coumarone-indene resins, phenol-modified coumarone-indeneresins, epoxy-modified coumarone-indene resins, etc.; terpene-basedresins such as α-pinene resins, β-pinene resins, polyterpene resins,hydrogenated terpene resins, aromatic modified terpene resins, terpenephenolic resins, etc.; petroleum-based resins such as aliphaticpetroleum resins, aromatic petroleum resins, aromatic modified aliphaticpetroleum resins, etc.; and so on. These can be used singly or incombination of two or more kinds. Particularly preferable arerosin-based resins, terpene-based resins, and coumarone-indene resins.

The tackifier is miscible with the rubber component, but immiscible withthe (meth)acrylic polymer. Thus, it is considered that with thetackifier being miscible with the rubber component, the modulus ofelasticity of the rubber phase decreases, and as a result, with theincreasing wettability to an adherend allows the adhesive strength toincrease.

The miscibility of a tackifier can be evaluated by the presence of hazein a PSA sheet after dried. More specifically, it is evaluated based onthe methods for “(2) Evaluation of the miscibility of rubber componentand tackifier” and “(3) Evaluation of the miscibility of (meth)acrylicpolymer and tackifier” in Examples described later.

The amount of the rubber-component-miscible tackifier to be added is,for instance, within a range above 0 part by weight up to 100 parts byweight, preferably 5 to 100 parts by weight, or more preferably 10 to 90parts by weight relative to 100 parts by weight of solid contents of therubber-based latex. When this amount is within this range, the modulusof elasticity of the rubber phase will be sufficiently reduced, and thewettability to an adherend will be increased to a greater degree. Sincethe glass transition temperature of the rubber phase will decrease andbecome able to suppress an increase in the modulus of elasticity at alow temperature, sufficient wettability to an adherend can be obtainedat a low temperature.

The rubber-component-miscible tackifier has a softening point ofpreferably 80° C. to 150° C. or more preferably 90° C. to 140° C. Whenit is below this range, the cohesive strength of the PSA will decrease.When it is above this range, the low temperature adhesive strength islikely to decrease.

It is noted that the softening point of a tackifying resin can bemeasured based on, for instance, the ring-and-ball method (BS K-5902).

Although the mode of addition of a tackifying resin is not particularlylimited, it is usually preferable that the tackifying resin is added ina water dispersion form where it is pre-dispersed in water. As such awater dispersion of a tackifying resin, a commercial product can beused. Alternatively, can be used a desired tackifier forced to dispersein water by a disperser.

In the water-dispersed PSA composition according to the presentinvention, in addition to the tackifier miscible with the rubbercomponent, may be used a tackifier (typically a tackifying resin)miscible with the (meth)acrylic polymer. By using such a tackifier incombination, can be produced an effect of further increasing theadhesive strength at room temperature.

Examples of such a tackifying resin miscible with the (meth)acrylicpolymer include rosin-based resins such as rosin esters, hydrogenatedrosin esters, disproportionated rosin esters, polymerized rosin esters,etc.; coumarone-indene-based resins such as coumarone-indene resins,hydrogenated coumarone-indene resins, phenol-modified coumarone-indeneresins, epoxy-modified coumarone-indene resins, etc.; terpene-basedresins such as α-pinene resins, β-pinene resins, polyterpene resins,hydrogenated terpene resins, aromatic modified terpene resins, terpenephenolic resins, etc.; petroleum-based resins such as aliphaticpetroleum resins, aromatic petroleum resins, aromatic modified aliphaticpetroleum resins, etc.; and so on. Rosin ester-based resins areparticularly preferable. These can be used singly or in combination oftwo or more kinds.

It is noted that as the tackifying resins of terpene-based, rosin-based,etc., various tackifying resins having a wide range of sp values areavailable (e.g., see “Handbook of Pressure-Sensitive Adhesive (3rd ed.)”(published by Japan Adhesive Tape Manufacturers Association)). A personof ordinary skill in the art may figure out how to obtain a tackifyingresin that is miscible with the rubber component, but immiscible withthe (meth)acrylic polymer based on the contents of description of thepresent application including the specific examples described later aswell as general technical knowledge such as sp values, etc.

The amount of the (meth)acrylic-polymer-miscible tackifying resin to beadded is, for instance, within a range greater than 0 part by weight upto 20 parts by weight or preferably 1 to 10 parts by weight relative to100 parts by weight of solid contents of the (meth)acrylic polymer. Whenit is above this range, the glass transition temperature of the acrylicphase formed primarily of the (meth)acrylic polymer increases and themodulus of elasticity at a low temperature increases, whereby, at a lowtemperature, sufficient wettability to an adherend may not be obtained,causing the adhesive strength to decrease.

In the water-dispersed PSA composition according to the presentinvention, in addition to the aforementioned tackifiers, may be used atackifying resin in a liquid state at room temperature.

Although the mode of addition of a tackifying resin in a liquid state atroom temperature is not particularly limited, the tackifying resin in aliquid state at room temperature is preferably added as a dispersionwhere it has been pre-dispersed in water. As a water dispersion of sucha liquid-at-room-temperature tackifying resin, a commercial product canbe used. Alternatively, can be used a desired tackifying resin forced todisperse in water by a disperser. Tackifying resin in a liquid state atroom temperature refers to a tackifying resin having a softening pointof 25° C. or below before dispersed as an emulsion. In the presentinvention, of these resins, low molecular weight polymers of rosinesters and/or terpene are particularly preferable.

The tackifying resin in a liquid state at room temperature is preferablymiscible with the rubber component. With the tackifying resin in aliquid state at room temperature being miscible with the rubbercomponent, the modulus of elasticity of the rubber phase decreases andthe wettability to an adherend increases, allowing the adhesive strengthto increase.

The amount of the liquid-at-room-temperature tackifying resin to beadded is, for instance, within a range greater than 0 part by weight upto 50 parts by weight or preferably 1 to 40 parts by weight relative to100 parts by weight of solid contents of the rubber-based latex. When itis above this range, the cohesive strength of the PSA will decrease.

(iv) Other Components

To the water-dispersed PSA composition according to the presentinvention, in accordance with its purpose and application, as necessary,a crosslinking agent can be added. Examples of the crosslinking agentinclude isocyanate-based crosslinking agents, epoxy-based crosslinkingagents, oxazoline-based crosslinking agents, aziridine-basedcrosslinking agents, metal chelate-based crosslinking agents, and so on.

These crosslinking agents are not particularly limited, and anoil-soluble or a water-soluble crosslinking agent can be used. Thesecrosslinking agents can be used singly or in combination, asappropriate. Its amount to be added is, for instance, 10 parts by weightor smaller, preferably 0.01 to 10 parts by weight, or more preferably0.02 to 5 parts by weight relative to 100 parts by weight of the(meth)acrylic polymer.

Examples of an isocyanate-based crosslinking agent include loweraliphatic polyisocyanates such as 1,2-ethylene diisocyanate,1,4-butylene diisocyanate, 1,6-hexamethylene diisocyanate, etc.;alicyclic polyisocyanates such as cyclopentylene diisocyanate,cyclohexylene diisocyanate, isophorone diisocyanate, hydrogenatedtolylene diisocyanate, hydrogenated xylene diisocyanate, etc.; andaromatic polyisocyanates such as 2,4-tolylene diisocyanate, 2,6-tolylenediisocyanate, 4,4′-diphenylmethane diisocyanate, xylylene diisocyanate,etc.; and so on. As the isocyanate-based crosslinking agent, forexample, can also be used commercial products including an adduct oftrimethylolpropane and tolylene diisocyanate (trade name “CORONATE L”available from Nippon Polyurethane Industry Co., Ltd.), an adduct oftrimethylolpropane and hexamethylene diisocyanate (trade name “CORONATEHL” available from Nippon Polyurethane Industry Co., Ltd.), an adduct oftrimethylolpropane and xylylene diisocyanate (trade name “TAKENATED-110N” available from Mitsui Chemicals, Inc.), and so on.

Examples of an epoxy-based crosslinking agent includeN,N,N′,N′-tetraglycidyl-m-xylenediamine, diglycidylaniline,1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, 1,6-hexanediol diglycidylether, neopentyl glycol diglycidyl ether, ethylene glycol diglycidylether, propylene glycol diglycidyl ether, polyethylene glycol diglycidylethers, polypropylene glycol diglycidyl ethers, sorbitol polyglycidylethers, glycerol polyglycidyl ethers, pentaerythritol polyglycidylethers, polyglycerol polyglycidyl ethers, sorbitan polyglycidyl ethers,trimethylolpropane polyglycidyl ethers, diglycidyl adipate, o-diglycidylphthalate, triglycidyl tris(2-hydroxyethyl)isocyanurate, resorcinoldiglycidyl ether and bisphenol-S diglycidyl ether; as well asepoxy-based resins having two or more epoxy groups per molecule. As theepoxy-based crosslinking agent, can also be used, for example,commercial products such as trade name “TETRAD C” available fromMitsubishi Gas Chemical Company, Inc., and so on.

Examples of an oxazoline-based crosslinking agent include those listedas examples in Japanese Patent Application Publication No. 2009-001673.In particular, it can be a compound having a main chain of an acrylstructure or a styrene structure as well as an oxazoline group as a sidechain of the main chain, or preferably an oxazoline-group-containingacrylic polymer having a main chain of an acryl structure and anoxazoline group as a side chain of the main chain.

Examples of an aziridine-based crosslinking agent includetrimethylolpropane tris[3-(1-azyridinyl)propionate] andtrimethylolpropane tris[3-(1-(2-methyl)azyridinyl propionate].

Examples of a metal chelate-based crosslinking agent include thoselisted in Japanese Patent Application Publication No. 2007-063536. Inparticular, examples include aluminum chelate-based compounds, titaniumchelate-based compounds, zinc chelate-based compounds, zirconiumchelate-based compounds, iron chelate-based compounds, cobaltchelate-based compounds, nickel chelate-based compounds, tinchelate-based compounds, manganese chelate-based compounds, and chromiumchelate-based compounds.

To the water-dispersed PSA composition according to the presentinvention, as necessary, can be added additives usually added to PSAsuch as thickeners, release adjusting agents, plasticizers, softeningagents, fillers, colorants (pigments, dyes, etc.), anti-aging agents,surfactants, leveling agents, anti-foaming agents, and so on. Theamounts of these additives to be added are not particularly limited andcan be suitably selected.

The thickener can be an acrylic alkaline thickening type, aurethane-based associative type, a clay-based one, a cellulose-basedone, a polyamide-based one, and so on. For example, it can be added inan amount of 0.01 to 1% by weight of the water-dispersed PSAcomposition. By using a thickener within such a range, thewater-dispersed PSA composition can be adjusted to have applicableviscosity and a PSA sheet free of air spots and grooves can be obtained.

The water-dispersed PSA composition according to the present inventionhas a storage modulus of 1 MPa or smaller over a range of −15° C. to 25°C., or preferably 0.9 MPa or smaller, when determined by a dynamicviscoelastic measurement in which shear strain is applied at a frequencyof 1 Hz after a PSA is formed. When it is above this range, thewettability to an adherend turns out insufficient and the adhesivestrength decreases. Although the lower limit is not particularlylimited, it can be, for instance, 0.01 MPa or greater.

It is noted that a person of ordinary skill in the art will be able toadjust the storage modulus at −15° C. to 25° C. determined by a dynamicviscoelastic measurement performed with shear strain at a frequency of 1Hz to be within the range described above by modifying the compositionbased on the contents of the present description including specificexamples described later as well as technical common knowledge.

(II) PSA Sheet

The PSA sheet according to the present invention comprises a PSA layerformed of the water-dispersed PSA composition described above.

The PSA sheet according to the present invention may have aconfiguration of an on-substrate PSA sheet where such a PSA layer isprovided on either face or each face of a substrate (support) sheet, orit may have a configuration of a substrate-free PSA sheet where the PSAlayer is held on a release sheet (which may be a substrate sheet havinga release surface). The concept of the PSA sheet referred to hereencompasses those called as PSA tapes, PSA labels, PSA films, and so on.

Although the PSA layer is typically formed in a continuous manner, it isnot limited to such a form and it can be formed into, for example, aregular or random pattern of dots, stripes, and so on. The PSA sheetprovided by the present invention may be in a roll or in a flat sheet.Alternatively, the PSA sheet can be processed into various other forms.

The PSA sheet according to the present invention can be made to have therespective cross-sectional structures schematically illustrated in FIG.3( a)-(c).

FIG. 3( a) shows a configuration example of an on-substrate PSA sheet ofan adhesively single-faced kind, in which PSA layer 2 is provided on oneface of substrate 1. For example, such a PSA sheet can be made into aroll by winding it so that, with substrate 1 having a release surface onits face opposite to the face having PSA layer 2, as shown in FIG. 3(a), the release surface of substrate 1 laminated with PSA layer 2 comesin contact with PSA layer 2 laminated on substrate 1.

The PSA sheet shown in FIG. 3( b) has a configuration where PSA layer 2is protected with release sheet 3 having a release surface at least onthe PSA layer side and it can also be made into a roll by winding it.

The PSA sheet shown in FIG. 3( c) has a configuration where PSA layer 2is provided on each face of substrate 1 and each of these PSA layers 2is protected with release sheet 3 having a release surface at least onthe PSA layer side. The PSA sheet shown in FIG. 3( c) can be made into aroll, for instance, by pre-forming PSA layers 2 on release sheets 3 andadhering these to each of the front and back faces of substrate 1, andwinding the resultant.

In the PSA sheet shown in FIG. 3( c), PSA layers 2 are provided on eachface of substrate 1. These PSA layers 2 may be formed of a PSA havingthe same composition or may be formed respectively of PSA's havingdifferent compositions.

Examples of a material forming the substrate include polyolefin-basedfilms such as polyethylenes, polypropylenes, ethylene-propylenecopolymers, etc.; polyester-based films such as polyethylene phthalate,etc.; plastic films such as polyvinyl chloride, etc.; papers such asKraft papers, Washi papers, etc.; fabrics such as cotton fabrics, staplecloth fabrics, etc.; non-woven fabrics such as polyester non-wovenfabrics, vinylon non-woven fabrics, etc.; and metal foils.

The plastic films may be non-stretched films, or stretched (uni-axiallystretched or bi-axially stretched) films. To the substrate surface to beprovided with a PSA layer, can be given a surface treatment such asprimer coating, corona discharge treatment, and so on.

The PSA layer can be obtained by applying the water-dispersed PSAcomposition described above to a substrate by a known coating methodfollowed by drying. The method for applying the water-dispersed PSAcomposition to a substrate is not particularly limited and it can becarried out using, for instance, a gravure roll coater, a reverse rollcoater, a kiss roll coater, a dip roll coater, a bar coater, a knifecoater, a spray coater, a fountain dye coater, a closed edge dye coater,and so on.

The PSA layer can be formed by transferring to a substrate a PSA layerpre-formed by applying the water-dispersed PSA composition to a releasesheet.

The thickness of the PSA layer after dried is not particularly limited,and it is, for instance, 500 μm or smaller, or preferably within a rangeof 5 μm to 200 μm. Although the drying temperature may depend on thekind of the substrate, it can be, for instance, within a range of 40° C.to 120═ C.

From the standpoint of providing a PSA sheet that exhibits goodadhesiveness to a non-polar adherend at a low temperature, the PSA sheetaccording to the present invention has preferably an adhesive strengthto a polypropylene (PP) plate at −5° C. of 3.0 N/20 mm or greater and anadhesive strength to a PP plate at −15° C. of 2.5 N/20 mm or greater,more preferably an adhesive strength to a PP plate at −5° C. of 3.5 N/20mm or greater and an adhesive strength to a PP plate at −15° C. of 3.0N/20 mm or greater, or even more preferably an adhesive strength to a PPplate at −5° C. of 4.0 N/20 mm or greater and an adhesive strength to aPP plate at −15° C. of 3.5 N/20 mm or greater.

From the standpoint of providing a PSA sheet that maintains itsproperties even at a low temperature, besides satisfying the adhesivestrength described above, it has preferably an adhesive strength to aSUS plate at −5° C. of 4.0 N/20 mm or greater and an adhesive strengthto a SUS plate at −15° C. of 4.0 N/20 mm or greater, more preferably anadhesive strength to a SUS plate at −5° C. of 4.5 N/20 mm or greater andan adhesive strength to a SUS at −15° C. plate of 4.5 N/20 mm orgreater, or even more preferably an adhesive strength to a SUS plate at−5° C. of 5.0 N/20 mm or greater and an adhesive strength to a SUS plateat −15° C. of 5.0 N/20 mm or greater.

EXAMPLES

The present invention is described more specifically with examples andcomparative examples below. However, the present invention is notlimited to the following examples and comparative examples. In thefollowing descriptions, “part” and “%” are based on the weight unlessotherwise specified.

Synthesis Example 1 Synthesis of a Water-Dispersed (Meth)Acrylic Polymer(A)

To a reaction vessel equipped with a condenser, a nitrogen inlet tube, athermometer and a stirrer, were added 96 parts of butyl acrylate (BA), 4parts of acrylic acid (AA), 0.08 part of t-dodecanethiol (chain transferagent), 2 parts of sodium polyoxyethylene lauryl sulfate (emulsifier)and 153 parts of ion-exchanged water as an emulsified mixture (i.e., anemulsion of starting monomers); and the resulting mixture was stirredunder nitrogen gas for one hour.

Then, it was heated to 60° C., and to this, was added, based on solidcontents, 0.1 part of2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate(polymerization initiator) (trade name “VA-057” available from Wako PureChemical Industries, Ltd.) prepared as a 10% aqueous solution, and thepolymerization was carried out for 3 hours. To this, was added 10%aqueous ammonia to adjust it to pH 7.5 and was obtained awater-dispersed (meth)acrylic polymer (A).

Synthesis Example 2 Synthesis of a Water-Dispersed (Meth)Acrylic Polymer(B)

In the same manner as Synthesis Example 1 except that 2-ethylhexylacrylate (2-EHA) was used in place of butyl acrylate (BA), was obtaineda water-dispersed (meth)acrylic polymer (B).

Example 1

Both based on solid contents, were mixed 70 parts of the water-dispersed(meth)acrylic polymer (A) obtained in Synthesis Example 1 and 30 partsof a synthetic polyisoprene latex (trade name “SEPOLEX IR-100K”available from Sumitomo Seika Chemicals Co., Ltd.) to prepare awater-dispersed PSA composition.

Subsequently, the resulting water-dispersed PSA composition was mixedwith 0.5 part of an acrylic alkali thickener (trade name “ARON B-500”available from Toagosei Co., Ltd.) to increase the viscosity. Thethickened water-dispersed PSA composition was applied to coat a 25 μmthick polyethylene terephthalate substrate (trade name “LUMIRROR S-10”available from Toray Industries, Inc.) so as to obtain a thickness of 60μm after dried, and this was allowed to dry at 100° C. for 3 minutes toprepare a PSA sheet.

Example 2

A PSA sheet was prepared in the same manner as Example 1 except that thewater-dispersed (meth)acrylic polymer (A) and the synthetic polyisoprenelatex (trade name “SEPOLEX IR-100K” available from Sumitomo SeikaChemicals Co., Ltd.) were mixed at a modified ratio shown in Table 4.

Example 3

Both based on solid contents, 70 parts of the water-dispersed(meth)acrylic polymer (A) obtained in Synthesis Example 1 and 30 partsof a synthetic polyisoprene latex (trade name “SEPOLEX IR-100K”available from Sumitomo Seika Chemicals Co., Ltd.) were mixed. To this,based on solid contents, was then added, as a tackifier, 6 parts (20parts relative to 100 parts of solid contents of the rubber latex) of anaromatic modified terpene resin emulsion (trade name “NANOLET R-1050”available from Yasuhara Chemical Co., Ltd., softening point 100° C.) toprepare a water-dispersed PSA composition.

Then, the resulting water-dispersed PSA composition was mixed with 0.5part of an acrylic alkali thickener (trade name “ARON B-500” availablefrom Toagosei Co., Ltd.) to increase the viscosity, and the thickenedwater-dispersed PSA composition was applied to coat a 25 μm thickpolyethylene terephthalate substrate (trade name “LUMIRROR S-10”available from Toray Industries, Inc.) so as to obtain a thickness of 60μm after dried and this was allowed to dry at 100° C. for 3 minutes toprepare a PSA sheet.

Examples 4, 6, 8-10, Comparative Examples 1-11

PSA sheets were prepared in the same manner as Example 3 except that thespecies and the ratios of (meth)acrylic polymer, rubber component andtackifier were as shown in Table 4.

Example 5

A PSA sheet was prepared in the same manner as Example 3 except that theamount of the tackifier was 18 parts (60 parts relative to 100 parts ofsolid contents of the rubber latex) instead of 6 parts and 0.035 part ofan epoxy-based crosslinking agent (trade name “TETRAD-C” available fromMitsubishi Gas Chemical Company, Inc.) was further added to prepare thewater-dispersed PSA composition.

Examples 7, 11

PSA sheets were prepared in the same manner as Example 5 except that thespecies and the ratios of (meth)acrylic polymer, rubber component andtackifier as well as the amounts of the crosslinking agent added were asshown in Table 4.

<Evaluations> (1) Evaluation of the Miscibility of (Meth)Acrylic Polymerand Rubber Component

Both based on solid contents, were mixed 70 parts of a water-dispersed(meth)acrylic polymer and 30 parts of a latex to prepare awater-dispersed PSA composition.

Then, the resulting water-dispersed PSA composition was mixed with 0.5part of an acrylic alkali thickener (trade name “ARON B-500” availablefrom Toagosei Co., Ltd.) to increase the viscosity and the thickenedwater-dispersed PSA composition was applied to coat a release sheet(trade name “DIAFOIL MRF-38” available from Mitsubishi Plastics, Inc.)so as to obtain a thickness of 60 μm after dried and this was allowed todry at 100° C. for 3 minutes to prepare a PSA sheet.

The PSA sheet was released from the release sheet and PSA layers werelaminated to prepare a laminate of about 2 mm thick and from this, a 7.9mm diameter measurement sample was cut out. The measurement sample wasplaced between 7.9 mm diameter parallel discs, and using a dynamicviscoelastic measurement device (“Advanced Rheometric Expansion System”available from Rheometric Scientific, Inc.), while applying shear strainat a frequency of 1 Hz, at a heating rate of 5° C./min, were measuredthe storage modulus (G′) and the loss modulus (G″) over a range of −70°C. to 100° C. Based on the storage modulus (G′) and the loss modulus(G″), the loss tangent tan δ was calculated by the following equation:

Loss tangent tan δ=G″/G′

The calculated loss tangent (tan δ) values were plotted against thetemperature to obtain a loss tangent curve and the presence of a(meth)acrylic polymer peak and a rubber component peak were checked.When the two peaks were not present, it was determined to be miscible(M) and when both of the peaks were present, it was determined to beimmiscible (IM). Table 1 shows the results of the evaluation of themiscibilities of the (meth)acrylic polymers and the rubber componentsused in Examples and Comparative Examples described above.

In addition, as an example, FIG. 2 shows a graph exhibiting the resultsof the evaluation on the miscibility of the (meth)acrylic polymer andthe rubber component used in Example 3.

TABLE 1 Water-dispersed (meth)acrylic polymer Trade name A B LatexIR-100K IM IM SB-0561 IM — SB-0589 IM — SB-0533 IM — SB-0568 IM —SB-2877A IM — MG-25 IM —

(2) Evaluation of the Miscibility of Rubber Component and Tackifier

To 100 parts of solid contents of the latex, was added, based on solidcontents, 20 parts of the tackifier to prepare a water-dispersed PSAcomposition. Then, the resulting water-dispersed PSA composition wasmixed with 0.5 part of an acrylic alkali thickener (trade name “ARONB-500” available from Toagosei Co., Ltd.) to increase the viscosity andthe thickened water-dispersed PSA composition was applied to coat arelease sheet (trade name “DIAFOIL MRF-38” available from MitsubishiPlastics, Inc.) so as to obtain a thickness of 60 μm after dried andthis was allowed to dry at 100° C. for 3 minutes to prepare a PSA sheet.

To one face of the prepared PSA sheet, was adhered a glass slide (“GlassSlide White Ground Edges” available from Matsunami Glass Ind., Ltd.; 1.3mm thick), and from this, the release sheet was removed to prepare asample for evaluation. The haze of this sample was measured with a hazemeter “HM-150” (available from Murakami Color Research Laboratory Co.,Ltd.), and based on JIS K7136, the value was calculated by the equation:haze (%)=Td/Tt×100 (Td: diffuse transmittance, Tt: total transmittance).When the calculated haze value was 50% or smaller (0 to 50%), it wasdetermined to be miscible and when it was greater than 50%, it wasdetermined to be immiscible.

It is noted that regarding the evaluation of the miscibility, withrespect to the prepared PSA sheet, it is preferable to determine it tobe miscible when its haze value is 45% or smaller and immiscible when itis greater than 45%, or it is more preferable to determine it to bemiscible when its haze value is 40% or smaller and immiscible when it isgreater than 40%.

Table 2 shows the evaluation results of the haze values with respect tothe pairs of the rubber component and the tackifier used in Examples andComparative Examples described above.

TABLE 2 Haze value (%) Tackifying resin R-1050 NS-100H Latex IR-100K 4.670.3 SB-0561 39.2 — SB-0589 2.2 — SB-0533 1.0 — SB-0568 1.8 — SB-2877A17.1 — MG-25 5.5 —

(3) Evaluation of the Miscibility of (Meth)Acrylic Polymer and Tackifier

To 100 parts of solid contents of the (meth)acrylic polymer, was added,based on solid contents, 20 parts of the tackifier to prepare awater-dispersed PSA composition. Then, the resulting water-dispersed PSAcomposition was mixed with 0.5 part of an acrylic alkali thickener(trade name “ARON B-500” available from Toagosei Co., Ltd.) to increasethe viscosity and the thickened water-dispersed PSA composition wasapplied to coat a release sheet (trade name “DIAFOIL MRF-38” availablefrom Mitsubishi Plastics, Inc.) so as to obtain a thickness of 60 μmafter dried and this was allowed to dry at 100° C. for 3 minutes toprepare a PSA sheet.

To one face of the prepared PSA sheet, was adhered a glass slide (“GlassSlide White Ground Edges” available from Matsunami Glass Ind., Ltd.; 1.3mm thick), and from this, the release sheet was removed to prepare asample for evaluation. The haze of this sample was measured with a hazemeter “HIM-150” (available from Murakami Color Research Laboratory Co.,Ltd.), and based on JIS K7136, the value was calculated by the equation:haze (%)=Td/Tt×100 (Td: diffuse transmittance, Tt: total transmittance).When the calculated haze value was 50% or smaller (0 to 50%), it wasdetermined to be miscible and when it was greater than 50%, it wasdetermined to be immiscible.

It is noted that regarding the evaluation of the miscibility, withrespect to the prepared PSA sheet, it is preferable to determine it tobe miscible when its haze value is 45% or smaller and immiscible when itis greater than 45%, or it is more preferable to determine it to bemiscible when its haze value is 40% or smaller and immiscible when it isgreater than 40%.

Table 3 shows the evaluation results of the haze values with respect tothe pairs of the (meth)acrylic polymer and the tackifier used inExamples and Comparative Examples described above.

TABLE 3 Haze value (%) Water-dispersed (meth)acrylic polymer A BTackifying resin R-1050 68.8 70.5 NS-100H 0.8 1.2

(4) Adhesive Strength

Each of the PSA sheets obtained in Examples and Comparative Examples wascut to a size of 20 mm by 100 mm, and under an atmosphere at ameasurement temperature (−5° C., −15° C.), this was adhered to, as anadherend, a SUS304 stainless steel plate or a polypropylene (PP) plate(trade name “PP-N-AN” available from Shin-Kobe Machinery Co., Ltd.) andpressure-bonded with a 2 kg roller moved back and forth once. Theresultant was then left under an atmosphere at a measurement temperature(−5° C., −15° C.) for 30 minutes, and in an atmosphere at −5° C. or −15°C., it was subjected to a peel test at a peeling angle of 180° and apeeling speed of 300 mm/min to measure the adhesive strength. Theresults are shown in Table 4.

(5) Dynamic Viscoelastic Measurement of PSA Sheet

Each of the water-dispersed PSA compositions obtained in Examples andComparative Examples was applied to coat a release sheet (trade name“DIAFOIL MRF-38” available from Mitsubishi Plastics, Inc.) so as toobtain a thickness of 60 μm an after dried and this was allowed to dryat 100° C. for 3 minutes to prepare a PSA sheet.

The PSA sheet was released from the release sheet and PSA layers werelaminated to prepare a laminate of about 2 mm thick and from this, a 7.9mm diameter measurement sample was cut out.

The measurement sample was placed between 7.9 mm diameter paralleldiscs, and using “Advanced Rheometric Expansion System” available fromRheometric Scientific, Inc., while applying shear strain at a frequencyof 1 Hz, at a heating rate of 5° C./min, the storage modulus (G′) wasmeasured over a range of −70° C. to 100° C. The results are shown inTable 4.

Although Table 4 shows only the storage modulus (G′) values at −15° C.and 25° C., with the PSA sheets obtained in the present Examples, asshown in FIG. 1, the G′ at −15° C. has the greatest value within a rangeof −15° C. to −25° C.; and therefore, if the G′ value at −15° C. is 1MPa or smaller, the G′ values over −15° C. to 25° C. are 1 MPa orsmaller.

TABLE 4 (Meth)acrylic Polymer (parts by weight of solid contents) RubberComponent Synthesis Synthesis (parts by weight of solid contents)Example1 Example 2 IR-100K SB 0561 SB 0589 SB 0533 SB 0568 SB 2877AMG-25 Ex. 1 70 30 Ex. 2 90 10 Ex. 3 70 30 Ex. 4 70 30 Ex. 5 70 30 Ex. 650 50 Ex. 7 50 50 Ex. 8 30 70 Ex. 9 70 30 Ex. 10 70 30 Ex. 11 70 30Comparative Ex. 1 100 Comparative Ex. 2 100 Comparative Ex. 3 90 10Comparative Ex. 4 90 10 Comparative Ex. 5 70 30 Comparative Ex. 6 70 30Comparative Ex. 7 70 30 Comparative Ex. 8 70 30 Comparative Ex. 9 70 30Comparative Ex. 10 70 30 Comparative Ex. 11 70 30 Tackifier (partsDynamic PP Adhesive SUS Adhesive by weight of Crosslinking AgentViscoelasticity Strength Strength solid contents) (parts by weight) G′(MPa) (N/20 mm) (N/20 mm) R-1050 NS-100H T/C 25° C. −15° C. −5° C. −15°C. −5° C. −15° C. Ex. 1 0 0.22 0.48 3.5 3.3 7.0 6.5 Ex. 2 0 0.19 0.603.6 3.2 6.8 7.6 Ex. 3 6 0.20 0.48 6.0 5.5 7.4 7.8 Ex. 4 12 0.20 0.50 5.25.6 6.7 7.8 Ex. 5 18 0.035 0.18 0.60 6.9 6.6 8.4 9.5 Ex. 6 10 0.25 0.475.2 6.5 6.0 8.0 Ex. 7 30 0.04 0.16 0.48 9.4 5.6 9.3 12.0 Ex. 8 14 0.270.38 6.7 8.2 6.2 8.1 Ex. 9 12 0.32 0.94 4.2 2.8 4.2 4.4 Ex. 10 12 3.50.2 0.8 7.1 6.2 7.3 8.9 Ex. 11 12 0.06 0.07 0.28 6.3 5.6 6.3 7.5Comparative Ex. 1 0.18 0.60 4.4 0.8 6.7 8.1 Comparative Ex. 2 20 0.163.31 1.3 2.0 11.2 2.5 Comparative Ex. 3 5 0.17 1.29 4.5 1.6 5.5 6.9Comparative Ex. 4 18 0.18 2.10 1.9 1.4 8.1 6.0 Comparative Ex. 5 14 0.241.20 7.1 1.8 8.2 9.8 Comparative Ex. 6 12 ≧10 ≧10 0.3 0.2 1.3 0.1Comparative Ex. 7 12 0.85 ≧10 0.8 0.2 1.3 0.1 Comparative Ex. 8 12 0.76≧10 0.5 0.2 2.2 0.2 Comparative Ex. 9 12 0.24 2.96 4.7 1.2 8.5 8.5Comparative Ex. 10 12 0.38 2.54 3.4 1.9 5.3 6.2 Comparative Ex. 11 140.32 3.9 2.7 0.7 4.4 3.8

It is noted that in Tables 1 to 4, “IR-100K” refers to a syntheticpolyisoprene latex (trade name “SEPOLEX IR-100K” available from SumitomoSeika Chemicals Co., Ltd.), of which the rubber component had a losstangent peak temperature of −53° C.; “SB0561” refers to astyrene-butadiene latex (trade name “SB-0561” available from JSRCorporation), of which the loss tangent peak temperature was −45° C.;“SB0589” refers to a styrene-butadiene latex (trade name “SB-0589”available from JSR Corporation), of which the loss tangent peaktemperature was 21° C.; “SB0533” refers to a styrene-butadiene latex(trade name “SB-0533” available from JSR Corporation), of which the losstangent peak temperature was 0° C.; “SB0568” refers to styrene-butadienelatex (trade name “SB-0568” available from JSR Corporation), of whichthe loss tangent peak temperature was −4° C.; “SB2877A” refers to astyrene-butadiene latex (trade name “SB-2877A” available from JSRCorporation), of which the loss tangent peak temperature was −26° C.;“MG-25” refers to a MMA-grafted natural rubber latex (trade name“REGITEX MG-25” available from Regitex Corporation), of which the losstangent peak temperature was −58° C.

In Tables 1 to 4, “R-1050”, “NS-100H”, and “T/C” refer to an aromaticmodified terpene resin emulsion (trade name “NANOLET R-1050” availablefrom Yasuhara Chemical Co., Ltd., softening point 100° C.), arosin-based tackifier (trade name “SUPER ESTER NS-100H” available fromArakawa Chemical Industries, Ltd., softening point 100° C.), and anepoxy-based crosslinking agent (trade name “TETRAD-C” available fromMitsubishi Gas Chemical Company, Inc.), respectively.

As shown in Table 4, the PSA sheets according to the present inventionwere confirmed to have an adhesive strength to a PP plate at −5° C. of3.0 N/20 mm or greater as well as an adhesive strength to a PP plate at−15° C. of 2.5 N/20 mm or greater, and exhibit good adhesiveness to anon-polar adherend at a low temperature.

Additionally, the PSA sheets according to the present invention wereconfirmed to have an adhesive strength to a SUS plate at −5° C. of 4.0N/20 mm or greater as well as an adhesive strength to a SUS plate at−15° C. of 4.0 N/20 mm or greater, and exhibit good adhesiveness also toa polar adherend at a low temperature.

The present invention is not limited to the respective embodimentsdescribed above and can be modified in a variety of ways within theranges described in the claims. The technical scope of the presentinvention encompasses embodiments obtained by suitably combiningtechnical means disclosed respectively in the different embodiments.

INDUSTRIAL APPLICABILITY

The water-dispersed PSA composition according to the present inventionis capable of forming a PSA that exhibits good adhesiveness to anon-polar adherend such as polyethylene and polypropylene at a lowtemperature. Therefore, it can be preferably used for a variety of PSAsheets.

REFERENCE SIGNS LIST

-   1 substrate-   2 PSA layer-   3 release sheet

1. A water-dispersed pressure-sensitive adhesive composition comprising:a (meth)acrylic polymer obtained by polymerizing a monomer compositioncomprising, as a primary component, an alkyl(meth)acrylate having analkyl group with 1 to 18 carbon atoms; and a latex containing a rubbercomponent that is immiscible with the (meth)acrylic polymer; a weightratio ((meth)acrylic polymer/rubber latex) of the (meth)acrylic polymerand the latex being within a range of 95/5 to 25/75 based on solidcontents, wherein a storage modulus in a range of −15° C. to 25° C. is 1MPa or smaller, the storage modulus being determined by a dynamicviscoelastic measurement in which shear strain is applied at a frequencyof 1 Hz, the measurement being taken with respect to the water-dispersedpressure-sensitive adhesive composition dried at 100° C. for 3 minutes.2. The water-dispersed pressure-sensitive adhesive composition accordingto claim 1, wherein the latex has a loss tangent peak temperature of −5°C. or below, the loss tangent peak temperature being determined by adynamic viscoelastic measurement in which shear strain is applied at afrequency of 1 Hz.
 3. The water-dispersed pressure-sensitive adhesivecomposition according to claim 1, further comprising a tackifier that ismiscible with the rubber component, but immiscible with the (meth)acylicpolymer.
 4. The water-dispersed pressure-sensitive adhesive compositionaccording to claim 3, comprising the tackifier in an amount within arange above 0 part by weight up to 100 parts by weight relative to 100parts by weight of solid contents of the latex.
 5. The water-dispersedpressure-sensitive adhesive composition according to claim 3, whereinthe tackifier has a softening point of 80° C. to 150° C.
 6. Thewater-dispersed pressure-sensitive adhesive composition according toclaim 1, wherein the rubber component is at least one species selectedfrom a group consisting of natural rubber, synthesized polyisoprenerubber, polybutadiene rubber, and styrene-butadiene rubber.
 7. Apressure-sensitive adhesive formed with the water-dispersedpressure-sensitive adhesive composition according to claim
 1. 8. Apressure-sensitive adhesive sheet comprising a pressure-sensitiveadhesive layer formed with the water-dispersed pressure-sensitiveadhesive composition according to claim
 1. 9. The water-dispersedpressure-sensitive adhesive composition according to claim 2, furthercomprising a tackifier that is miscible with the rubber component, butimmiscible with the (meth)acylic polymer.
 10. The water-dispersedpressure-sensitive adhesive composition according to claim 9, comprisingthe tackifier in an amount within a range above 0 part by weight up to100 parts by weight relative to 100 parts by weight of solid contents ofthe latex.
 11. The water-dispersed pressure-sensitive adhesivecomposition according to claim 9, wherein the tackifier has a softeningpoint of 80° C. to 150° C.
 12. The water-dispersed pressure-sensitiveadhesive composition according to claim 2, wherein the rubber componentis at least one species selected from a group consisting of naturalrubber, synthesized polyisoprene rubber, polybutadiene rubber, andstyrene-butadiene rubber.
 13. A pressure-sensitive adhesive formed withthe water-dispersed pressure-sensitive adhesive composition according toclaim
 2. 14. A pressure-sensitive adhesive sheet comprising apressure-sensitive adhesive layer formed with the water-dispersedpressure-sensitive adhesive composition according to claim
 2. 15. Thewater-dispersed pressure-sensitive adhesive composition according toclaim 4, wherein the tackifier has a softening point of 80° C. to 150°C.
 16. The water-dispersed pressure-sensitive adhesive compositionaccording to claim 3, wherein the rubber component is at least onespecies selected from a group consisting of natural rubber, synthesizedpolyisoprene rubber, polybutadiene rubber, and styrene-butadiene rubber.17. A pressure-sensitive adhesive formed with the water-dispersedpressure-sensitive adhesive composition according to claim
 3. 18. Apressure-sensitive adhesive sheet comprising a pressure-sensitiveadhesive layer formed with the water-dispersed pressure-sensitiveadhesive composition according to claim
 3. 19. The water-dispersedpressure-sensitive adhesive composition according to claim 10, whereinthe tackifier has a softening point of 80° C. to 150° C.
 20. Thewater-dispersed pressure-sensitive adhesive composition according toclaim 4, wherein the rubber component is at least one species selectedfrom a group consisting of natural rubber, synthesized polyisoprenerubber, polybutadiene rubber, and styrene-butadiene rubber.